Drop-down menu background clearness

Hi there,
How do I change the background darkness/clearness from left to right?
I'm using RTL language and that's why I need the background color orientation to swap, so my menu links will be on the darker side of the menu.
See picture attached

Thank you for your help

Image is:

templates/hot_cosmetics/images/topmenu_dropdown_bg.png

All image editors have option "Flip Horizontally", just apply that on image